There are a number of communication links, such as HF transmission paths and disturbed satellite communication links, that may introduce distortion or contamination into messages transmitted over the link and thereby make it difficult to faithfully reproduce the original message at the receiver. This distortion (including loss of signal) may be the result of any number of effects, such as multi-path reception, group delay distortion, noise amplitude interference, dispersive fading or, in general, a smearing or time-spread of the time response of the communication link. Because of these adverse influences, the data received is seldom the same as the original message, so that some form of anti-distortion compensation must be employed in the receiver's signal processing equipment.
Such anti-distortion compensation typically involves the use of an adaptive equalizer (such as a decision feedback equalizer) through which the distortion-introducing characteristics of the channel are estimated and through which the received signal is modified to compensate for the distortion of the channel. Because the characteristics of the channel are dynamic, it is necessary to continually update the settings (e.g. weighting coefficients) of the equalizer. For this purpose, it has been found that by interspersing blocks of known symbols within the data message being transmitted, the equalizer may continually compare the known data bursts in the received signal with a stored replica and adjust itself to correct for differences in the stored replica and received bursts, so that it is able to avoid drifting due to large bursts of error (such as severe time-varying intersymbol interference introduced by the channel) and/or signal fading. This technique is described in U.S. Pat. No. 4,365,338 to D. D. McRae et al entitled "Technique for High Rate Digital Transmission Over a Dynamic Dispersive Channel", issued Dec. 21, 1982 and assigned to the Assignee of the present application, and reference may be had to that patent for a detailed description of such a signal formatting and processing scheme.
As is described in the above-referenced patent and as is commonly employed in signal recovery systems employing an equalizer as part of the signal correction process, in order for the equalizer to adapt itself to the incoming signal, it is necessary for the receiver to lock-onto the carrier and align itself with the symbol rate of the incoming data. This is typically achieved through the use of a synchronization preamble, such as a prescribed tone transmitted for a preselected time interval followed by a phase reversal of the tone. Through this sync preamble the carrier recovery loop and symbol rate clock are able to lock onto the incoming signal and provide timing control for the equalizer, so that it may proceed to carry out its own task of compensation which requires that the replica of a known symbol burst (e.g. a PN code) be synchronized with the known data (PN code) in the received signal upon which the equalizer operates. While the sync preamble enables the receiver to achieve synchronization between the transmitted and reference known data bursts, in order for the receiver to recover from a severe and/or extended signal fade, the sync preamble often must be retransmitted, thereby reducing available bandwidth.